<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3492: Minimal improvements to elements_view::iterator</title>
<meta property="og:title" content="Issue 3492: Minimal improvements to elements_view::iterator">
<meta property="og:description" content="C++ library issue. Status: C++23">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3492.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++23">C++23</a> status.</em></p>
<h3 id="3492"><a href="lwg-defects.html#3492">3492</a>. Minimal improvements to <code>elements_view::<i>iterator</i></code></h3>
<p><b>Section:</b> 25.7.23.3 <a href="https://wg21.link/range.elements.iterator">[range.elements.iterator]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Michael Schellenberger Costa <b>Opened:</b> 2020-10-28 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>0
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.elements.iterator">active issues</a> in [range.elements.iterator].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.elements.iterator">issues</a> in [range.elements.iterator].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
During code review of <code>elements_view</code> for MSVC-STL we found two issues that should be easily addressed:
</p>
<ol>
<li>
<p><code>elements_view::<i>iterator</i></code> constraints both <code>operator++(int)</code> member functions</p>
<blockquote><pre>
constexpr void operator++(int) requires (!forward_range&lt;<i>Base</i>&gt;);
constexpr <i>iterator</i> operator++(int) requires forward_range&lt;<i>Base</i>&gt;;  
</pre></blockquote>
<p>
However, given that a constrained method would be preferred we only need to constrain one of those. 
The proposal would be to remove the constraint from the <code>void</code> returning overload and change 
the declaration to
</p>
<blockquote><pre>
constexpr void operator++(int);
constexpr <i>iterator</i> operator++(int) requires forward_range&lt;<i>Base</i>&gt;;  
</pre></blockquote>
</li>
<li>
<p><code>elements_view::<i>iterator</i> operator-</code> is constrained as follows:</p>
<blockquote><pre>
friend constexpr difference_type operator-(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  requires random_access_range&lt;<i>Base</i>&gt;; 
</pre></blockquote>
<p>
However, that requires its base to have <code>operator-</code> defined. We should change the constraint to 
<code>sized_sentinel_for&lt;iterator_t&lt;<i>Base</i>&gt;, iterator_t&lt;Base&gt;&gt;</code>:
</p>
<blockquote><pre>
friend constexpr difference_type operator-(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  requires sized_sentinel_for&lt;iterator_t&lt;<i>Base</i>&gt;, iterator_t&lt;<i>Base</i>&gt;&gt;;
</pre></blockquote>
</li>
</ol>

<p><i>[2020-11-01; Daniel comments]</i></p>

<p>
Bullet (2) of the discussion has already been resolved by LWG <a href="lwg-defects.html#3483" title="transform_view::iterator's difference is overconstrained (Status: C++23)">3483</a><sup><a href="https://cplusplus.github.io/LWG/issue3483" title="Latest snapshot">(i)</a></sup>, it has therefore
been omitted from the proposed wording below.
</p>

<p><i>[2020-11-15; Reflector prioritization]</i></p>

<p>
Set priority to 0 and status to Tentatively Ready after five votes in favour during reflector discussions.
</p>
<p><i>[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3492"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4868">N4868</a>. 
</p>

<blockquote class="note">
<p>
This wording intentionally only touches <code>operator++(int)</code> and <em>not</em> <code>operator-</code>,
see the 2020-11-01 comment for the reason why.
</p>
</blockquote>

<ol>
<li>
<p>
Modify 25.7.23.3 <a href="https://wg21.link/range.elements.iterator">[range.elements.iterator]</a>, class template <code>elements_view::<i>iterator</i></code> synopsis, 
as indicated:
</p>

<blockquote>
<blockquote>
<pre>
[&hellip;]
constexpr <i>iterator</i>&amp; operator++();
constexpr void operator++(int) <del>requires (!forward_range&lt;<i>Base</i>&gt;)</del>;
constexpr <i>iterator</i> operator++(int) requires forward_range&lt;<i>Base</i>&gt;;
[&hellip;]
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr void operator++(int) <del>requires (!forward_range&lt;<i>Base</i>&gt;)</del>;
</pre>
<blockquote>
<p>
-6- <i>Effects:</i> Equivalent to: <code>++<i>current_</i></code>.
</p>
</blockquote>
<pre>
constexpr <i>iterator</i> operator++(int) requires forward_range&lt;<i>Base</i>&gt;;
</pre>
[&hellip;]
</blockquote>
</li>
</ol>





</body>
</html>
